Commentary: Aliso Creek should be left to meander

February 27, 2013|By Elisabeth M. Brown | By Elisabeth M. Brown

Seen from the air, the Mississippi snakes across the terrain in thousands of curves and loops. The landscape is dominated by circular and semicircular arcs, the remains of old meanders. Cutoff loops become ponds or lakes.

A river's length and shape are determined by a number of factors. Topography and volume of water are the most important. In its mountainous headwaters, a river is fast-moving; rushing down-slope, rough with rapids, carrying large boulders. Approaching the valley floor, the water slows in the more gradual terrain. As it loses energy, it drops the biggest rocks first, then ever-smaller ones. Eventually, it flows sluggishly, carrying only fine silt.

In nearly flat country, a river meanders through the landscape in large twists and turns. Meanders greatly widen the flood plain, the land subject to periodic flooding. This is favored farmland, fertile with the nutrients of deposited silt and easy to till.

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When land near a river is urbanized, people want to control the river to prevent further flooding. The Army Corps of Engineers builds and maintains miles of levees along the banks of the Mississippi.

In Orange County, with much smaller rivers and creeks, "flood control" means lining riverbanks and channel bottoms with concrete, straightening the channels and narrowing them. This deepens the mass of rushing water and increases its speed and scouring power. More than half the length of Aliso Creek is subject to these man-made controls.

Aliso Creek drains a watershed of 30-plus square miles. Today it is 19 miles long, from the Santa Ana Mountains to the Pacific Ocean. Originally, it was more like 21 miles in length. Hydrologists say that if you shorten a river by straightening, it will try to get its missing miles back downstream.

The last few miles of the creek run through Aliso and Wood Canyons Wilderness Park, where the effects of channeling and increased water flow from urban runoff play out dramatically.

Through the park, Aliso Creek is down-cutting, and can be 15 feet below its banks. Where we should see a vigorous streamside community of willows and mulefat, there are often only steep, eroding earthen banks. A small dam was built in the 1980s to raise the water level and irrigate planted willows, but the planting failed.

At every curve the creek erodes the banks of the channel with its energy. This exposes the underground pipes of the South Orange County Wastewater Authority sewage treatment plant. The answer is more concrete on the channel walls.

By the time it reaches the Aliso Canyon golf course, the frustrated creek spills over its banks, given half a chance. More than once, mud has covered the golf course for many weeks. In the early 1990s, the creek carried bright white clay from a toll road construction site in the foothills where Route 241 crosses El Toro Road and deposited it on the golf course.

The county's defunct Super Project and SOCWA's proposed Sludge Pipeline answer with more expensive concrete solutions. But a river is a dynamic system responding to the physical laws of water flow. Engineers talk about taming rivers with dams and levees, but ultimately they can't be changed, only accommodated. It's time to look at giving Aliso Creek back some of its missing two miles by letting it meander.